An antenna is a device that transmits electrical signals into free space. The signals may be, for example, received by another antenna located at a proximate or a distant location. The antennas may be mounted within, for example, a transmission device in a wireless communication network. Some examples of transmission devices include wireless base station or access point devices, and mobile station devices.
The Institute of Electrical and Electronic Engineers (IEEE) has established a wireless local area network (LAN) standard, 802.11. The IEEE 802.11 standard (IEEE 802.11) outlines Media Access Control (MAC) and Physical Layer (PHY) specifications for wireless LANs. The specification of the IEEE 802.11 addresses transmission of data in wireless LANs. In particular, the IEEE 802.11a standard addresses communication in wireless asynchronous transfer mode (ATM) systems, covering frequencies of operation between 5 gigahertz (GHz, or 109 Hz) and 6 GHz. As is known in the art, IEEE 802.11a requires a modulation method called orthogonal frequency-division multiplexing (OFDM), which allows communication to occur at extremely high data speeds by transmitting data over multiple frequency channels over a wide frequency range.
The IEEE 802.11a specification takes into account successful and unsuccessful transmission of packets, for example data packets, and includes mechanisms designed to thwart problems with packet transmission, such as requiring in-order transmission of packets, and retransmission by a transmitting entity of packets that were not received properly by a receiving entity. As with the Internet protocol (IP) and the Ethernet protocol, no error correction codes are permitted in IEEE 802.11a. Since a packet that is sent by a source and that is received in error at, or missed by, a destination must be retransmitted by the source in order to obtain the correct packet at the destination receiver, retransmission of packets is a central aspect of IEEE 802.11a access points and mobile stations.
It is not unusual for an antenna to receive a signal across a fading channel. Multiple antennas are typically used in communication systems to provide another option to turn to, in the event of poor signal reception due to a fading channel, so that a good channel with no fading can be found. Some examples of causes of a fading channel include phase shift in the signal and multipath interference errors. The RF energy that is transmitted between antennas can experience destructive and constructive interference due to multiple paths taken by the energy with multiple delays on the way to a receive antenna. The interference can cause a receive antenna to receive a packet in error or to miss a packet entirely.
Ideally, antenna diversity techniques are used when a particular channel is fading due to multipath effects so that changing from one antenna to another antenna provides another communication channel that in all probability is not fading.
Traditionally, fast antenna diversity techniques have been used to manage multiple antennas. As an example of this approach with two antennas, when a packet arrives, a first antenna is used to receive the signal. After receiving for a sufficient period of time to judge reception quality, the radio is switched to a second antenna. The second antenna is then used to receive the signal until the quality of reception can be judged. Finally, the radio is switched to using whichever of the antennas had the best reception. In some cases, more than two antennas are used.
Trying and testing multiple antennas using fast antenna diversity typically takes place during a preamble, header, or training portion of the packet. The preamble is examined rather than the data so that no data are lost while the different antennas are being tested.
There are several reasons why this approach is undesirable for the IEEE 802.11a standard, and for any other high data rate radio system. First, the packet preamble in IEEE 802.11a is quite short, for example, eight microseconds total duration. A short preamble is desirable in any high data rate communications system in order to keep the efficiency of the communications system high. As data rates get higher, the duration of standard Ethernet length packets get shorter. If the preamble is a long period in time, then the efficiency is low. While having a short preamble is good for efficiency, the short preamble reduces the time available to test using both, or all, of the antennas. Switching between antennas takes a certain time based on the physical constraints of driving electrical switches. In addition, there is a minimum time needed to measure the signal from a given antenna to effectively determine the quality of the signal. When the measurement time (i.e., the duration of the preamble) is very short, a very poor estimate of the quality may be obtained.
Time that is consumed switching and measuring the signals from different antennas reduces the amount of time available to perform other functions that commonly need to be performed during the packet preamble. These functions may include, for example, correctly setting the gains of amplifiers in the receive chain, extracting the frequency offset of the incoming signal, and finding the proper symbol boundaries. When the preamble is short, the quality of the frequency offset, gain setting, or symbol timing may be compromised if time is spent selecting the best antenna. For these reasons, in practice, the IEEE 802.11a preamble is too short to allow testing of multiple antennas. If one were to try to force antenna selection into the time of the preamble, one would actually degrade the overall performance of the communications system and would typically make poor choices of which antenna to use.
Fast antenna diversity switching during the packet preamble creates an additional challenge that is unique to the IEEE 802.11a OFDM system. Since the OFDM signal is a wideband signal, the differences in performance between two omni-directional antennas will not be the total receive power. More likely, the differences between two antennas will be the narrow notching of certain narrow frequency ranges due to multipath interference.
However, the IEEE 802.11a preamble consists of a relatively few, widely spaced narrow frequencies. Therefore, when considered as a test signal, the IEEE 802.11a preamble cannot be used to sense many of the narrow notches within the narrow frequency bands that might occur due to multipath interference. Testing during the IEEE 802.11a preamble would thus be detrimental to the reception of the normal data portion of the packet, which will span all of the frequency ranges. In IEEE 802.11a systems, it is problematic to apply known approaches, such as sensing the channel using a packet preamble, to make the correct choice between two antennas.
Typically, antenna diversity techniques assume that the decision process has access to signals from two or more antennas. Many of these techniques are based on examining an average of a combination of signals from two or more antennas. One averaging method is ark maximum ratio combining where signals coming from two or more antennas are assigned different weights and are added together to form a weighted combination signal.
In contrast to IEEE 802.11a systems, most wireless systems are narrowband signal systems. Narrowband signals are generally thought of in terms of having signal bandwidths of hundreds of kilohertz (kHz, or 103 Hz), for example, 500 kHz or 1 megahertz (MHz, or 106 Hz), or less, depending on the transmitting and receiving channel response. Wideband and broadband signals are generally thought of in terms of having signal bandwidths above 1 MHz depending on the transmitting and receiving channel response. In IEEE 802.11a systems, the signals have operating frequencies in the neighborhood of between 5 and 6 GHz, so these signals are clearly wideband signals by any definition.
In narrowband systems, because the bandwidths are limited to hundreds of kHz or less, two or more receive signals from two or more respective antennas generally do not show significant variations, i.e. the signals have a relatively flat response, within the signal bandwidth relative to each other. This means that the two or more signals can be combined rather easily using an antenna diversity combining technique with little risk of losing information by deviating from the true signal or of the received signals cancelling each other out. Generally, the amplitude and phase responses of narrowband signals do not vary as significantly across antennas as with a wideband or broadband signal. A narrowband signal from one antenna may be slightly attenuated due to a fading channel but the attenuation typically does not cause problems in combining the antenna signal with a narrowband signal from another antenna.
In a broadband or a wideband signal system such as an IEEE 802.11a compliant communications system at frequencies around 5 GHz, however, the amplitude and phase responses of received signals can be expected to vary widely from each other across multiple antennas. In fact, it can be shown that given sufficient separation distance between antennas, the received signals are completely independent of each other. Combining completely independent signals is troublesome as the signals may cancel each other out. More generally, wideband or broadband RF signals cannot easily be summed up, at least not using traditional combination methods and conventional narrowband diversity techniques. Although antenna diversity techniques are well-known, these techniques are not applicable to a 5 or 6 GHz wideband application.
It is possible to envision brute force implementations of normal fast antenna diversity combining techniques to a 5 GHz system that would imply massive processing capability to combine every subchannel separately. The signals would need to be separated into sections having a narrow bandwidth of hundreds of KHz, for example, sections of 300 kHz bandwidth to drill down to a level at which the variations between the signals would be negligible for purposes of combining the signals. The signals would then have to be combined separately at each section—implying separate combining logic at every section—which would achieve the desired result, but at great complexity and at a huge cost. In effect, normal diversity techniques are prohibitively expensive in an IEEE 802.11a compliant environment, unless the performance benefits derived from the techniques can justify the cost—a situation that is hard to envision in the overwhelming majority of circumstances.
Needed are cost effective antenna diversity techniques that are suited to confront the unique challenges posed by high data rate systems such as IEEE 802.11a compliant systems.